ARRGH!! Teaching ampacity to 'Stuborn Folks!'

Status
Not open for further replies.
B

brother

Senior Member
Location
"I have learned, in whatsoever STATE I am, therewi
Well, I'm sure this isn't the first, but it won't be the last either. There's this other electrician that insist that its ok to use the 90degree ampacity column for the THHN THWN-2 when installing the wires. He's been told that you have to go with the 75 degree column because of the terminals on the breakers, devices etc...

Unless he has all equipment rated at the 90 degree Celsius (in which I have NEVER seen any). He's also been told that the 90 degree Celsius is only good for derating because you start at the larger ampacity, but in no case can you use it above the 75 degree Celsius because of the breakers rating.

He's been doing this for a long time, and it just scares me when I see stuff like this, and people just dont want to learn the right way to do things.

Ok, I'm done venting, at least so far there hasn't been any fires/burnt buildings when people do make that mistake, Maybe there should be a national law (not just a state law) for training on electricity before letting people do this.
 
I've noticed all breakers have a 60/75 degree rating; all 30A and above receptacles I've come across have dual rating; and all commercial grade receptacles and switches that I have seen and use are dual rating.

Those 39-cent Orange Box devices are only rated 60 degrees as far as I know.
 
He's also been told that the 90 degree Celsius is only good for derating because you start at the larger ampacity, but in no case can you use it above the 75 degree Celsius because of the breakers rating.
Click to expand...

He's never wired a a/c unit has he?
 
480sparky said:
So what, exactly, is this 'other electrician' doing wrong?
Click to expand...

Ok, Ill try to explain more,

Lets take #6 awg copper for example. If you look at the table 310.16 in the 90 degree column its good for 75 amps. In the 75 degree column its only good for 65 amps. This guy would install the #6 and use a 70 amp breaker, (we are not dealing with motors or any other exception here). He will argue since #6 is good for 75 amps in the 90 degree column because of the insulation, so he says he can use it there.

We try to tell him He cannot do that because of the breakers rating is only good up to 75degrees and you have to go with the 65 amp rating of #6, not the 75 amp. The breaker is oversized or the wire is undersized. I hope this helps clear it up.
 
brother said:
Ok, Ill try to explain more,

Lets take #6 awg copper for example. If you look at the table 310.16 in the 90 degree column its good for 75 amps. In the 75 degree column its only good for 65 amps. This guy would install the #6 and use a 70 amp breaker, (we are not dealing with motors or any other exception here). He will argue since #6 is good for 75 amps in the 90 degree column because of the insulation, so he says he can use it there.

We try to tell him He cannot do that because of the breakers rating is only good up to 75degrees and you have to go with the 65 amp rating of #6, not the 75 amp. The breaker is oversized or the wire is undersized. I hope this helps clear it up.
Click to expand...

Ok, thats correct but in your first post the wording wasnt clear. Sorry
 
I agree 100% with brother that the 90 degree column is for derating purposes only, unless all of the "links in the chain" (terminals, conductors, breakers, etc) have the 90 degree rating.

However, in you example of #6 AWG CU on a 70 amp breaker...

doesn't 240.4 (B) allow us to size our breaker to the next higher standard size above the ampacity of the conductors? (60 and 70 being standard sizes, of course)


Great forum by the way.
 
ceb58 said:
He's never wired a a/c unit has he?
Click to expand...

Even if it is a motor we still must use the 60 or 75 bracket. What we are allowed to do is select a higher breaker for protection but the wire is still protected because of internal overload . Yes higher amp breaker but not because of 90 degree rating.
 
rclacey said:
I agree 100% with brother that the 90 degree column is for derating purposes only, unless all of the "links in the chain" (terminals, conductors, breakers, etc) have the 90 degree rating.

However, in you example of #6 AWG CU on a 70 amp breaker...

doesn't 240.4 (B) allow us to size our breaker to the next higher standard size above the ampacity of the conductors? (60 and 70 being standard sizes, of course)


Great forum by the way.
Click to expand...

Yes a 65 amp conductor would permitted to be protected at the next standard size OCPD which is 70 amps if the connected load is 65 amps or less.
 
brother said:
Ok, Ill try to explain more,

. . .
Click to expand...
Okay, when you said "He's been told . . . " we thought you meant by whoever he first heard it from, but now we know you meant he's been told that by you.

Yes, you're correct about the temperatures; you just happened to pick the #6 which fell between two breaker sizes and started the above side discussion.
 
LarryFine said:
Okay, when you said "He's been told . . . " we thought you meant by whoever he first heard it from, but now we know you meant he's been told that by you.

Yes, you're correct about the temperatures; you just happened to pick the #6 which fell between two breaker sizes and started the above side discussion.
Click to expand...

Ok, Maybe I should have used #4 as an example instead of #6. Sorry for the confusion. He would also use the 90 degree column for the 'load' calculation as well. For example, if he has a load of 92 amps, he would still use #4 because of the 90 degree column its good for 95 amps. He has been told by other electricians that he would have to use the 85 amp rating for the #4 because of the 75 degree rating of the breakers.
 
Status
Not open for further replies.
Top